A locally thick blank was employed for the stamping of thin high strength steel sheets to increase the wall thickness at the inner corner of stamped products. To form the locally thick blank, a 2-stage local thickening process was developed for increasing the thickness of a blank at the portion equivalent to the inner corner. In the 1st stage of the local thickening process, a ring including the portion equivalent to the inner corner is drawn, and the drawn ring is compressed with the flat tools in the 2nd stage to attain a local increase in wall thickness. The 2-stage local thickening process was designed from both finite element simulation and an experiment. A locally thick blank having an increased wall thickness of 8% was formed, and the wall thickness at the inner corner of the stamped wheel disk from the locally thick blank was successfully increased by 9.6%. It was found that the locally thick blanks are attractive for controlling the distributions of the wall thickness of stamped products from thin high strength steel sheets.

View full abstract

he effect of hot shear spinning on the elimination of casting defects such as shrinkage cavities and dendrites in cast aluminum alloys was examined. Hot shear spinning is effective for achieving a large plastic deformation under a comparatively low forming load. Although many casting defects were formed in the cast blank, the shrinkage cavities and dendrites were successfully eliminated at an ironing ratio of above 20 %. The tensile strength and elongation of the formed parts were improved by eliminating the casting defects by hot shear spinning. The shear spinning process was simulated by the three-dimensional finite element method, and the casting defects were eliminated above an equivalent plastic strain of 1. It was found that hot shear spinning is effective for improving the mechanical properties of cast aluminum alloy parts.

View full abstract

The resemblance between frictional contact problems and elastoplasticity theory is mentioned in order to implement frictional contact problems in the finite element method. The equations used for regularity in frictional contact problems are defined. Then, forming simulations of surface coated steels are performed using a finite element program based on the dynamic explicit method in which a nonlinear friction model is implemented. First, a sliding friction test is simulated to check whether this implementation is successful. Secondly, deep drawing with a rear member die set is carried out to estimate the efficiency of the nonlinear friction model in the actual forming. From these verification, results of numerical simulations using nonlinear friction model strongly coincide with the experimental results.

View full abstract

A shaft of quenched steel and a thick flange of mild steel were successfully joined by forming a hole in the flange with serrations provided on the shaft. The effects of forming depth and contact angle on deformation during joining and on the strength of the joint obtained are discussed in this paper. The filling-up of serrations formed in the flange increased with increasing contact angle and forming depth. However, very large contact angles caused fracture and lowered the filling-up ratio. Thus, the optimum contact angle that maximized the filling-up ratio without fracture was determined, which was approximately 30° at a forming depth of 0.5mm. Under this condition, the hardness of the formed serrations and the joining force were also higher. A good correlation was found between the filling-up ratio and the joining strength as long as no cracking occurred during joining.

View full abstract

The pressure vessel for a reactor is composed of a body (straight shell) and a panel. Recently, the diameter of shells has increased because there is a tendency for pressure vessels to be designed on increasingly large scales. As a result, a tapered spacer is inserted between the shell and the panel. However, this increases the cost because of the need for welding. Thus, a solid shell, which is composed of a tapered spacer and a straight shell, is required. In this study, a new forging method for the fabrication of a solid shell was developed on the basis of a model experiment, and the deformation behavior was formulated from the result of the model experiment evaluating the forging process.

View full abstract

A new method (Q_t method) of gaining a higher time resolution of a shock wavefront structure is proposed for polycarbonate (PC) at particle velocities up to 1 km/s. The modified unsteady wave sensing system (M-UWSS), which we proposed before, using the Q₁ method instead of the conventional Q₂ method on the basis of plate impact experiment with three in-material PVDF gauges is applied to the construction of shock stress-strain curves up to the Hugoniot stress σ_H and the Hugoniot strain ε_H. However, piezofilm thickness has a considerable effect on wavefront structure. Thus, we derive from basic equations of piezoelectricity that the charge release per unit area is proportional to the ratio of the thickness of the shocked region to the total thickness of the piezofilm. It is demonstrated that the rise time of the shock charge density q in the piezofilm induced by such a shock in the Q₂, Q₁ and Q_t methods, in this order, is markedly decreasing. Among the three methods, the latest Q_t method has the highest accuracy.

View full abstract